EDB — 0H9

[0H9] Topics:boundary.Difficulty:*.

Given $X$ topological space and $A\subseteq X$; if $A$ is open (or closed) the boundary $\partial A$ has empty interior; we have $\partial A\supseteq \partial \partial A$ with equality if $\partial A$ has empty interior; in addition $\partial \partial A=\partial \partial \partial A$.